We have studied relaxation and recombination processes in compressed, doubly polarized atomic hydrogen at temperatures from 0.13 to 0.60 K and magnetic fields from 3 to 9 T. The gas and surface dipole three-body recombination rate constants at a field of 7.6 T are measured to be, respectively,
Lg=8.9(8)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}39}$
cm6 ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$ and
Ls=1.2(4)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}24}$
cm4 ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$. They decrease slowly with field and
Ls exhibits no significant angular dependence. The three-body recombination rate due to hyperfine mixing has also been measured. Electronic and nuclear relaxation rates have been measured; the b-c electronic relaxation rate constant in the gas is
Gbc=1.03(7)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}15}$
cm3 ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$exp(-
Ebc/
kBT). The temperature and field dependence of the nuclear relaxation rate in the gas are observed to be in excellent agreement with recent theoretical calculations. Three-body surface recombination-rate measurements using $^{3}\mathrm{\ensuremath{-}}^{4}$He surfaces indicate that as little as one monolayer of
3He on the liquid
4He surface appreciably decreases the adsorption energy of atomic hydrogen. Densities achieved include 4.5\ifmmode\times\else\texttimes\fi{}
1018 atoms${\mathrm{?}}^{\mathrm{\ensuremath{-}}3}$ at 0.55 K (pure
4He walls), and 1.4\ifmmode\times\else\texttimes\fi{}
1018 atoms${\mathrm{?}}^{\mathrm{\ensuremath{-}}3}$ at 0.19 K (8 at. ? sup 3---He).
Spin-polarized atomic hydrogen (H↓) gas at low temperatures (<0.5 K) has been studied using Electron Spin Resonance (ESR) techniques which allow direct measurements of the densities of H atoms in each of the two lower hyperfine states (a and b). It has been demonstrated that ESR can be made to yield accurate and detailed information about the decay of H↓ densities in time. Crucial to the analysis of the data is the ability to confine the sample of H↓, gas inside the liquid ⁴He coated microwave cavity where the magnetic field and temperature are well known. Measurements of K[sub aa] and K[sub ab], the two surface recombination rate constants, and their ratio γ=K[sub aa]/K[sub ab] have been made and from these results the binding energy of H on liquid ⁴He has been extracted. Also reported are the results of the first detailed study of the one-body surface relaxation rate which we find is due to microscopic magnetic impurities in the cell walls. Theoretical calculations of the two-body dipole-dipole relaxation rate of H↓ in the gas and on the surface are also presented.
In this thesis we describe the results of two sets of experiments on spin-polarized atomic hydrogen at temperatures below 1 K using a 115 GHz heterodyne ESR spectrometer. First, we have made measurements of the rates of the two-body recombination processes in spin-polarized atomic hydrogen gas and spin-polarized atomic deuterium gas in a 41 kG field. In the second set of experiments, we examined the ESR absorption line associated with the spin-polarized atomic hydrogen atoms adsorbed on surfaces of liquid helium. In hydrogen, a comparision of the measured recombination rates in 41 kG with the corresponding rates measured in different magnetic fields showed that an additional recombination process is activate for the conditions of these experiments. We demonstrate that this is due to a resonant recombination process via the (v,J) = (14,4) level of molecular hydrogen. In spin-polarized deuterium, we found that the recombination rate are much larger than the analogous rates in hydrogen, and therefore that atomic deuterium gas is much less stable than the hydrogen system under comparable conditions. We report the first observation and study of doubly spin-polarized deuterium, a gas in which both the deuteron and electron spins are aligned. At temperatures near 0.1 K, the absorption line of doubly spin-polarized H atoms changes drastically as a peak appears on both sides of the peak associated with the bulk atoms. We have shown that these new peaks are due to the atoms which are adsorbed onto the surface of the liquid helium film that coats the walls of the microwave cavity. The geometry dependent non-zero average magnetic dipolar field due to aligned spins on a planar surface results in a shift of the position of the ESR absorption lines. The lineshapes of the side peaks are very unusual and are best described as ramp-like. Even after extensive attempts to explain this lineshape, we still do not know what mechanism is responsible.
We have studied spin-polarized deuterium, ${\mathrm{D}}_{\ensuremath{\downarrow}}$, in the temperature range 0.3 to 0.7 K in a field of 40 kG using electron-spin resonance at 114 GHz. The intrinsic rates for recombination of two D atoms to form
D2 on the surface of $l\ensuremath{-}^{4}\mathrm{He}$ have been measured for both the ortho and para channels. The two rates are approximately equal and when extrapolated to zero field are 1600 times greater than the rates for
H2 formation. Doubly spin-polarized deuterium, ${\mathrm{D}}_{\ensuremath{\downarrow},↡}$, has been observed for the first time, with nuclear-spin purities > 0.98 and sample lifetimes as long as 1 h.
We calculate the surface dipolar recombination rate
Ls for spin-polarized hydrogen adsorbed on
4He surfaces at temperatures in the 0.2- to 0.6-K regime and for magnetic fields up to 30 T. For a magnetic field of 7.6 T normal to the surface and 0.4 K we find ${L}_{s}=1.3(3)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}25}$
cm4 ${\mathrm{s}}^{\ensuremath{-}1}$ increasing by 10%/T in the range of experimental interest. The anisotropy with the direction of the magnetic field is considerably smaller than in the case of the surface dipolar relaxation.
We analyze and reinterpret experiments with electron- and nuclear-spin (``doubly'')-polarized hydrogen (H\ensuremath{\downarrow}?) in a cell with a very low concentration of magnetic impurities. The destabilizing magnetic relaxation due to remaining impurities (
Gis) is found to be suppressed by increasing the thickness of the helium film covering the walls of the cell. Using recent information on surface three-body recombination, we obtain good quantitative agreement with more recent experiments if we assume thermal gradients, up to \ensuremath{\Delta}T/T\ensuremath{\simeq}0.1. We extract both the value and field dependence for the three-body surface recombination [
L3s=1.5(2)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}24}$
cm4 ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$ at B=8 T]. We also extract the magnetic field dependence of the impurity relaxation.